A BRIEF EXCURSION INTO THREE AGRICULTURAL REVOLUTIONS

by

Donald G. Baker
Department of Soil, Water, and Climate
University of Minnesota
St. Paul, Minnesota

INTRODUCTION

The word revolution used in the title indicates that three
sudden and radical changes occurred to agriculture. I believe you
will agree that the changes were radical but two of the three
were far from sudden. So, obviously, use of the word revolution
is applied with a certain license.

In each of the three so-called revolutions, technology and
climate played major roles. The technology in the form of an
implement, an instrument or a process creating the revolution is
usually relatively easy to cite. However, the part climate played
is more difficult to pinpoint. Climate is like civilization
itself, we can look back upon it and recognize changes that have
occurred, but definitive explanations as to causes and timing
remain elusive.

With this proviso - - let us proceed.

(This document is still under construction. Figures will be
added as we receive permission and as time is available.
References to sources of most of the documents not locally
produced are listed at the end if you wish to see them before we
can make them available here.)

1. Pre-History

Previous to the domestication of plants, man was a hunter and
a gatherer and most probably stored little food. As determined by
anthropologists who tested their theories on wild plants in the
mid-east, Fig. 1, where progenitors of our modern small grains
still exist, a gatherer more or less unconsciously seeks plants
that have larger seeds, more seeds per ear, and a compact
inflorescence (Evans, 1980). The loose inflorescence of oats and
the relative difficulty in gathering it may explain why oats was
probably the last of the small grains to be domesticated, Fig. 2.

The main point I wish to make is that man was measuring yield,
consciously or unconsciously, in terms of yield per human effort
or yield per time involved. This then can be considered as the
first yield expression.

2. The First Agricultural Revolution

The domestication of plants, possibly the greatest single
milestone in man's history, Fig.3, is generally accepted as the
point where plants retained their seed upon maturity. Thus,
threshing is required as well as the intentional sowing of the
grain, since the succeeding crop no longer occurs as a result of
the natural shedding of the seed due to movement of the plant,
for example, by the wind or passing animals. Over time, yields
have been expressed in three different ways and a fourth may come
into use in the near future. They are shown in Table 1.

Table 1. Crop yield expressions and approximate
period of use.

EXPRESSION

PERIOD

Yield/human effort*

Pre-Domestication of plants

Yield/seed sown

3-8,000 B.C. to 1000-1500 A.D.

Yield/land area

1000-1500 A.D. to present

Yield/critical factor

future use?

The first agricultural revolution and the associated dawn of
civilization apparently coincided with the warming of the earth
centered around 5-6000 years ago, Fig. 4, following the end of
the Pleistocene, the last "ice age". The ice age ended
about 8-10,000 years ago. The climate change that occurred
following this ice age was a definite improvement and undoubtedly
played an important part in this first revolution. If for nothing
else, it meant that attention could be devoted to more than
keeping the "home" fires or perhaps the
"cave" fires burning.

With plant domestication it was no longer possible to rely on
naturally scattered grain for regeneration of the crop. Sowing
became essential along with the necessary self-discipline
required to hold some grain aside as seed for the next year.
Along with this self-discipline came a greater awareness of the
weather and its powerful influence on crop yields as noted, for
example, in the famous Biblical story of Joseph and his plans for
the 7 full years and the 7 years of famine (according to Biblical
scholars occurring between 2000-1700 B.C.) . Sacrifices to the
gods were no longer sufficient. In other words the domestication
of plants also brought about a domestication of man, a civilizing
influence, if you wish (Evans, 1980). Ever since agriculture and
civilization have gone hand in hand.

With agriculture, a whole new set of conditions came into
being, generally associated with the dawn of civilization, Table
2, and a new criterion of determining yield was established. That
is, yield of grain compared to the amount sown. This is the
measure mentioned in the Bible, also by Roman writers, and by the
ancient Chinese. At the time of Christ wheat yields were about 3
or 4 to 1, but on good, fertile soils they could be considerably
higher (Evans, 1980). By comparison, the average Minnesota wheat
yield today is approximately 100 to 1. Although this comparison
is not exactly fair, since the seed of modern wheat plants is
also larger and may have other advantages as well. The return
with commercial corn today is about 800-1000 to 1.

Agricultural implements initially may have been weapons which
served a secondary purpose as a tool to scratch the surface. From
these rudimentary beginnings there developed the hoe and the
plow. The plow is considered to be the most important
agricultural implement since the beginning of history. First
there was a foot plow, shown at the top of Fig. 5, also called
"digging sticks" and used as shown in Egyptian tomb
paintings, Fig. 6, then the "ard plow", also termed a
"traction plow," Fig. 7, which was at first human
powered and required two to service it as illustrated, Fig. 8.
(There is some discussion whether these instruments should be
termed plows, since they lack two parts, the colter and the
moldboard, often used to differentiate a plow from similar
instruments.) A somewhat advanced "ard plow" is
illustrated in a Chinese drawing, Fig. 9, dated about the sixth
century A.D.

Table 2. Events of the first agricultural
revolution.

CLIMATE

POST-GLACIAL WARM PERIOD

WILD PLANTS

DOMESTICATED PLANTS

HUNTERS AND GATHERERS

GATHERERS AND HUNTERS

NOMADIC LIFE

SEDENTARY LIFE

WILD ANIMALS

DOMESTICATED ANIMALS

WEAPONS

TOOLS

LOCATION

TIGRIS AND EUPHRATES RIVERS

OCCURRENCE

5 - 8,000 BC

YIELD EXPRESSION

YIELD/HUMAN EFFORT

SEARCH FOR BETTER LANDS (?)

YES, AS INDICATED BY SETTLEMENT
LOCATIONS

3. The Second Agricultural Revolution

The next method of yield measurement introduces us to the
second great agricultural revolution and provides us with a
fascinating story that few people even in agriculture are aware
of. Like the first revolution, it rests upon a favorable climate
and upon two special features - an improved plow and the horse.
Together these three, climate, plow, and horse, created a
revolution in agriculture that can be compared to the post-World
War II agricultural revolution, though it extended over a much
longer period. This second revolution took place beginning about
500 or 600 A.D. (Burke, 1978) and is centered upon the Medieval
period. It is little known because of a peculiarity of many
historians that is only belatedly being corrected. For too long
historians wrote with a view limited to man's conflicts and his
artistic and literary accomplishments. The fact that technology
and those who contribute to it are seldom recognized is a
phenomenon of long standing in the western world (Gimpel, 1976).
This was noted by Plato, and even Leonardo da Vinci felt the
scorn of the intellectuals of his day who considered him little
better than a manual worker or technician. The existence of these
two groups, that is, the literary or artistic individual and the
scientist or technician existing side by side but with virtually
no contact, was a frequent subject of the late English author
C.P. Snow. He wrote of the gulf existing between the two groups,
or "cultures" in his terminology, and noted that the
distance between Greenwich Village in New York and the
Massachusetts Institute of Technology in Boston is like an ocean.
As a matter of fact members of Greenwich Village, New York, are
much closer intellectually to Chelsea, London's home of the arts,
while the M.I.T. staff in Boston are closer to South Kensington,
London's science center, than they are to each other (Gimpel,
1976). In other words the literary or artistic types simply
neither understand nor appreciate the scientist and technician.

a. The "Dark Ages"

The historian of the past seldom considered the technology or
science of an age with more than a passing reference. For this
reason we have the Medieval period dismissed until recently as
the Dark Ages. In fact it was anything but that, except perhaps
immediately after the breakup of the Roman Empire. (In this
regard it is interesting to read what the Encyclopedia
Britannica, 1945 printing, has to say about this period:
"Dark Ages was a term formerly used to cover the whole
period between the end of classical civilization, [that is, Greek
and Roman period] and the revival of learning in the 15th
century. Use of the term [Dark Ages] implied an exclusive respect
for classical standards in literature and art and a corresponding
disparagement of all that was achieved between the decline of
ancient culture and the work of Renaissance scholars, writers,
and artists."). In many respects the Medieval period has
outshone even the Renaissance, which the conventional historian
had convinced us was the real flowering of man's intellect. The
foundations of our present technologically oriented society were
not laid in the Italian Renaissance or the English Industrial
Revolution but in the Medieval period.

Based on the generally accepted view of medieval man it is
difficult to realize that he was surrounded by machines (Gimpel,
1976). Water power was developed as it had not been by the Greeks
and Romans. The Domesday Book of William the Conqueror records a
total of 5624 water mills in the England of 1086 A.D. There
apparently were two reasons for the lack of mills in Rome and
Greece. One was the dependence upon slaves. The second reason,
and perhaps the more important of the two, is the Mediterranean
climate which does not provide sufficient summer precipitation
for a constant stream flow, as noted in Fig. 10,
comparing monthly precipitation at Athens and Rome with that of
the Twin Cities.

Water power in the Medieval period was not always limited to a
static power source as, for example, a mill adjacent to a dam.
Sometimes the mills were mounted on barges permitting their
movement as either business or stream flow varied. Medieval man
also made use of wind power. Windmills may have been introduced
from the plateaus of ancient Persia (modern Iran and Afghanistan)
(Gimpel, 1976). Wind was a power source well adapted to the level
plains of northern Europe. Again this was a power source little
used if at all in Greece or Rome. Finally and most surprisingly
was the use of tidal power, an energy source currently under
investigation by our Department of Energy, or at least it was
during the energy emergency of the 1970's.

Medieval man was an artisan in the broadest sense, that is, an
artist and a mechanic. Churches such as the great cathedrals of
Europe having stained glass windows such as those of Sainte
Chapelle, Paris, Fig. 11, or the south aisle of Winchester
Cathedral, England, Fig. 12, which were built essentially within
the period 1180 to 1250 A.D. (Clark, 1948). They show better than
any words the skill and artistry of the craftsmen. The
engineering and technical knowledge demonstrated by these
structures is that of accomplished engineers and artisans, they
cannot be the product of a "dark age," while the
development of power sources demonstrates the mechanical ability
of Medieval man. Modern historians are now having to revise
history, and the term Dark Ages has been dropped in favor of the
Medieval period, subdivided into Early, Middle, and Late.

b. The Climate

The conventional historian also overlooked a number of factors
relative to man's agricultural progress in the period from about
600 or 800 to 1200 A.D. First, it was a period that was
climatically advantageous for agriculture, Fig. 13. In fact, it
is known as the "Medieval Climatic Optimum" because the
climate was both warmer and drier than it had been for some time
either before or afterwards. This warmer climate may also have
reduced forest expansion or even caused it to retreat. Because of
the higher temperatures, crops could be grown at greater
elevations. For example, in northern England during the World War
II food emergency plowing campaign of 1940-44 elevations were
reached which had not been under the plow since the Medieval
period (Gimpel, 1976).

c. The Implements

As already noted, the first agricultural tools were probably
the hoe followed by the foot and "ard" plows. Then came
the Mediterranean "scratch" or swing plow, Fig. 14. The
scratch plow, essentially a sharpened stick with handles for
guidance and a pole for attachment to an animal or a human, was
adapted to light or coarse textured soils. It is shown in this
delightful wooden model of oxen and plow found in an Egyptian
tomb of about 2000 B.C., Fig. 15. It may even have been in use
three to four thousand years before Christ. In order to prepare
an adequate seedbed with this plow it was necessary to crisscross
the field as shown in Fig. 16. As a result the field shape was
usually square.

A statement attributed to Daniel Webster can be interjected at
this point: "In tillage is the beginning of all art."
Since the presence of art implies that there is a civilization,
several important ideas can be gleaned from this statement.
First, agriculture, as evidenced by tillage, not only requires
skill but is an obvious manifestation of civilization. Today it
is not as common as it once was to define an artist as "one
versed in the practice of a fine manual occupation, as
sewing." Nor was it common to restrict the definition of art
to only the fine arts. We have only to consider the former names
of some universities to remember that art and an artist can be
broadly defined: Michigan State and Iowa State universities once
carried the name Agricultural and Mechanic Arts, just as Texas A
and M and Oklahoma A and M do today. And up to a few years ago a
major high school in St. Paul was named Mechanic Arts High
School. Even architecture was taught there.

Sometime in the sixth century A.D. a different plow appeared
which carried two extremely important features: a knife called a
"colter," which could cut through heavy roots, and a
mold-board which lifted the cut soil to one side, Fig. 17. These
two features serve to define a plow and separate it from other
instruments. These features, the knife and the moldboard, wrought
major changes in Medieval agriculture, especially in combination
with the horse and the very essential horse collar which probably
entered the scene a century or two later.

Wet fields could be plowed and the furrows running the full
length of a field improved the drainage. With this plow deeper
rooted vegetation could be removed and the heavier (finer
textured) more inherently fertile soils of northern Europe could
also be worked. The previously forested lands were now entered,
and they became the "new lands" of the Medieval period.
It is of interest that most European cities with "new"
in their name, such as Neuchatel, Switzerland, and Neumunster,
Germany, were formed at this time. Indeed, chronicles describing
the removal of forests and the settling of people in northern
Europe, have been compared to the stories of the opening of the
American West (Burke, 1978). A word was even developed to
describe what was taking place in France at that time. It was
"essart", now spelled "assart," (which can be
found in unabridged English dictionaries) which means the
grubbing up of the trees.

This new plow, in addition to being heavier, was also longer
as more than two animals were frequently used to pull the plow.
This necessitated a major change. For more efficient use of the
plow, the shape of fields was changed from square to rectangular,
Fig. 18, since even with a front wheel, Fig. 19, and Fig. 20,
shown in some Medieval illustrations (Burke, 1978), the plow was
obviously more difficult to handle at turns than the
"scratch" or swing plow. Another change wrought by this
plow was the development of cooperatives (Burke, 1978). The plow,
together with the animals to power it, represented major
investments. Thus, in most cases the investment required could
only be accomplished through the peasants banding together in
cooperatives and sharing ownership.

Because the long and narrow fields could no longer be readily
criss-crossed, the furrows made were not acceptable seedbeds so
the harrow was soon developed. Examples of harrows include three:
the first used brush held in place by stones, Fig. 21, the second
consists of wooden spikes, Fig. 22, even though it was in use in
early 19th century, and the last a roller which performed a task
similar to the harrow, Fig. 23. I found this roller in
southwestern France a few years ago. Upon close inspection, I
discovered it consisted of a Roman column.

d. The Horse Collar

Probably of equal importance to the heavy wheeled plow was the
development of the horse collar. A collar similar to one used on
camels was introduced from the east, perhaps from Bactria
(ancient Afghanistan), reaching Europe around 800 or 900 A.D.
This device permitted the exploitation of the horse. It is hard
to believe but neither the Greeks nor Romans (who represented the
"Classic period" for historians) were able to fully
exploit the horse because only a variation of the ox yoke was
originally used on the horse. A yoke suitable for oxen is shown
in Fig. 24 (from a Russian book). The yoke not only pressed on
the jugular vein of the horse, but it succeeded in choking the
horse if the load were more than about 1000 pounds. In fact the
Theodosian code of 438 A.D., the Roman Law under Emperor
Theodosius, decreed that a horse should not pull a load greater
than 1000 pounds. It was not until 1910 that a French cavalry
officer tested this weight limit and determined that a horse
would indeed choke if forced to pull a load of that size using a
yoke (Gimpel, 1976). Fig. 25 illustrates a modern horse collar.
Lovers of Greek art celebrate the genius of Greek sculptures
because the horse looks so "noble". Actually the
"noble" horse, with its head held high, did so to
prevent choking itself.

The Romans had also failed to harness horses so they could
work in line. For example, Roman chariots were pulled by two,
three, and occasionally four horses with the horses always
abreast, never in line. It is interesting to know, too, that the
Romans were apparently slow to develop a four-wheeled wagon in
which the front axle could be swiveled. If true this probably
explains the straight road system that they developed. It appears
that the Romans either adopted or reinvented a wagon in which the
front axle could be turned, Fig. 26, an invention of the first
century B.C. attributed to the Celts (Williams, 1987). With the
horse collar and new harness the number of horses in line, not
abreast, could now be increased.

Another event that aided in the exploitation of the horse was
the development of the lowly horse shoe and nails, Fig. 27. This
obviously helped greatly and permitted field work to be done
under a wider range of soil and weather conditions, since the
shoe gave greater traction and helped prevent hoof rot.

With the introduction of the horse collar, horseshoe, and
nails, a remarkable series of events occurred once the horse
could be truly exploited, Table 3. The horse, in contrast to the
ox, is 50% faster and has greater endurance, working two to three
more hours per day (Gimpel, 1976). Thus, in a sense, the limiting
factor became the amount of land that could be farmed. Therefore,
the new way to express yields became yield per unit land area,
bushels or pounds per acre, and so on.

Because more land could now be exploited, the 2-field Roman
system of one field fallow and the other in crop was replaced by
the 3-field system. With 3 fields only one-third of the land was
now in fallow, thus releasing more land for crops. Another
advantage of the 3-field system was that a greater variety of
crops was possible with a marked dietary improvement. Thus, with
two plantings and two harvests a better distribution of labor and
a decreased susceptibility to weather and crop losses became a
part of the new agriculture (Gimpel, 1976). A further change was
the nearly universal growth of oats as a crop for the horses.

It was about the time of this new plow that the unit of land
called an acre came into use. It was defined as the amount of
land that one horse or two oxen could plow in one day.

The so-called "climatic optimum" which more or less
coincided with and was in part responsible for the advances made
during the Medieval period ended abruptly. The succeeding century
was simply miserable. Not only did the climate become cold and
wet in Europe but the 14th century ushered in a hideous famine
(1315-1317), the Hundred Years' War (1338 - 1453), the Black
Death (1347-1350), and a series of peasant revolts in England as
well as the Continent.

Table 3. Events of the Second Agricultural
Revolution (Medieval Period)

CLIMATIC WARM PERIOD (1700-1200 A.D.)

STICK PLOW

MOLDBOARD PLOW (6TH CENTURY)

WHEELED PLOW

HORSE COLLAR (9th CENTURY)

HORSE

1. 50% Faster

2. Greater Endurance

3. Horseshoe + nails

4. Hitch in line

YIELD -- Yield Per Unit Area

SOCIAL CHANGES

1. Cooperatives

2. Villages Formed

3. Population Increase

PHYSICAL CHANGES

1. Field Drainage

2. Harrow

3. Field Shape

4. 3-Field System

a. Diet Improvement

b. Spread of Risk

c. More Land

5. Finer Texture Soils Can Be Worked

6. Deforestation

7. Oats a Universal Crop

4. The Third Agricultural Revolution

From today's vantage point it is hard to believe that there
was little change in agriculture from the Medieval period until
about the middle of this century. Sure, tractors were taking over
from the horse, and the binder, reaper, and threshing machine
were reducing the work required. Nevertheless, the U.S. horse
population didn't reach its peak until 1914, and crop yields were
not all that much better than they were in the Medieval period of
about 700 years earlier. Nor had tillage methods changed much: as
the "minute man's" plow of 1775 shows us, Fig. 28, and
as shown in these 20th century scenes: Fig. 29, an
"ard" plow still used in Italy, the scratch plow in
Ecuador, Fig. 30, and two scenes from Korea in 1951, Fig. 31, and
Fig. 32. The third scene from Korea, Fig. 33, illustrates a
Korean horse of diminutive stature. The small size is due either
to malnutrition or a purposeful breeding to obtain miniature
horses for children of the imperial family some centuries ago.

This third revolution was the most abrupt of the three. The
delay in the application of the accumulated technology was caused
by World War II and then the Korean Conflict. But the effects of
this revolution are immediately apparent when viewing yield data.
Yields throughout the advanced countries, with England's wheat
yields as an example, Fig. 34, show a similar post World War II
major increase in yield.

It is my contention that a particular climatic anomaly is in
part responsible for the recent economic problems faced by
Midwestern agriculture in particular. The long term corn yield
record of Minnesota, Fig. 35, will be used to demonstrate this.
The first portion, 1866-1938 shows a yield averaging only about
30 bushels per acre, which is not much better than a very good
yield in the Medieval period (if corn had been grown then). This
was followed by a 42 bushel per acre average yield from 1939-1951
when some of the new technology such as commercial fertilizers
and hybrid corn began to be applied. This was then followed by a
yield trend, 1952-present, that has shown an increase equaling
nearly 2 bushels per acre per year as technology became fully
adopted. It can be assumed that the trend lines are due to
technology (or lack of it) and that the variation about the three
trend lines is due to weather. In a very real sense the yield of
a crop, as illustrated here, represents an integration of the
climate for a given season. In other words, yield can upon
occasion serve as proxy evidence of climate.

In Fig. 36 is shown the variation of the annual yields about
the trend line. The yield depressions in 1976 and 1988, both
drought years, and the wet season of 1993, are very evident. But
when viewed not in absolute terms as departures from the
appropriate trend line, but as a percentage variation from the
general trend lines, Fig. 37, it becomes evident that the yield
depressions of 1934 and 1936 were relatively more serious for
farmers of that time than the yield depressions of 1976 and 1988
were for today's farmers.

According to this long term yield record the usual state of
affairs is seen to be one with large yield variations due to a
"hostile" climate, while the "benign" climate
from about the late 1940s to the early 1970s, with decreased
yield variation, was the unusual feature, an anomaly. The heart
of this "benign" climate period was about 1952-1964 as
shown in Fig. 38, which is based on the variation of annual
temperatures for 13-year periods. It shows just how rare this
"benign" period was. It was truly an anomalous event.
In other words, the modern farmer should plan for the expected
climate, which is one of large yield variation from year to year.

It is my belief that this "benign" period helped
bring about an enthusiasm, and indeed a general euphoria which
became evident in the 1960s among Midwestern farmers in
particular and in U.S. agricultural circles in general. This
optimism and a willingness to take greater risks in terms of
investment in land was increasing, especially among the younger
farmers who had experienced no major adversity up to this point.
They were too young to have experienced the 1930s. After all, the
weather has been remarkably tranquil during much of their tenure,
and yields increased almost every year as the application of
technology increased. So unique was this "benign"
period that some "experts" felt that technology had
even overcome the effects of weather. As a result, the climate,
the general economy, and even technology were setting agriculture
up for a dizzying ride, first up and then down.

Unbeknownst to all but the keenest of observers was the fact
that the "benign" climate had ended in the early 1970s
and was returning to the more usual or "hostile"
climate of former years. The first indication of this in
Minnesota was the early frost of 1974, and three years of
declining precipitation that culminated in the drought of 1976.

Soon to follow and a natural culmination of these events was
the abrupt contraction of land values. And with a more
"hostile" climate, that is, a more variable one, it
meant that yields and, therefore, income were no longer reliable
and the high land prices could no longer be supported. Thus, the
agricultural depression of the 1980s was ushered in.

5. A Fourth Agricultural Revolution?

The third revolution may run its course or it may receive a
boost from biotechnology. But with or without the application of
a new technology, a fourth method of yield measurement may be
used in the near future. It is the ratio of yield to a critical
factor other than land. As the critical factor in the past has
gone from human effort, to the amount of seed sown, to the amount
of land used, it may soon change, for example, to the nitrogen,
the phosphorus, or the energy expended. Perhaps the best one
would be an economic one, since it also requires a superior
bookkeeping system. Thus, the next yield expression might become
yield per dollar spent.

Figure Captions

1. The Near East where civilization apparently began. This
area now comprises the modern states of Israel, Jordan, Syria,
southeastern Turkey and Iraq. It is also known as the
"Fertile Crescent". (From p. 59 of Smith, 1995).

2. The inflorescences left to right of barley, oats, and
wheat.

3. The seven primary centers of agricultural development with
the approximate time periods when plants and animals were first
domesticated. (From p. 13 of Smith, 1995.)

4. The average air temperature variations for the past 18,000
years. (From Fig. 184., p. 481, Ahrens, 1994.)

18. The medieval wheeled plow with knife (colter) and
moldboard and the typical rectangular field associated with this
plow. (From p. 63 of Burke, 1978.)

19. The medieval heavy wheeled plow with knife (colter) and
moldboard with four oxen and three plowmen in 11th Century
Britain. (Fig. 13, p. 84 of Curwen and Holt, 1953.)

20. A print of agricultural life in medieval Britain. In
foreground is the heavy, wheeled plow and just above it is the
essential harrow. (From p. 64 of Burke, 1978.)

21. The bush harrow. Note the log to hold the brush and
branches in place. (From p. 114 of Vince, 1983.)

22. A wooden drag harrow used in the early 19th Century in
Britain. (From Fig. 24 of Lawson, 1982.)

23. A heavy stone roller used to serve the same purpose as a
cultipacker. Photographed in France in 1976. The stone roller is
actually a Roman column.

24. An oxen yoke, upper right, attached to a scratch or swing
plow. (From Fig. 53, p. 92 from Krasnov, 1987; a Russian book on
early agricultural implements. I am unable to provide book title
as it is entirely in Russian.)

25. The modern horse collar shown on a team of plowing horses.
(From p. 96 of Hall, 1992.)

26. Wagon

27. Horseshoes and nails. (From Fig. 128 of Lawson, 1982.)

28. The "Minute Man" of Lexington and Concord who
met the call to arms in 1775. Appropriately there is a simple
plow (without colter or moldboard) at his side.

29. A team of oxen pulling an ard plow in modern Italy. (From
Fig. 8, p. 75 of Curwen and Hatt, 1953.)

38. The 13-year running standard deviations of the eastern
Minnesota average annual temperatures. A 13-year period was
selected because it is the length with the minimum standard
deviation of the many lengths tested. (From Baker et.al.,1993.)